Use of Tellurium for inclusion modification:
The use of Te for improving the machinability of sulphur containing steel has been in practice for several years. The basic effect of Te is to globularize the inclusions leading to better deformation characteristics during hot working of steel. The ratio [WTe]/[WS] has been found to be important. A [WTe]/[WS] ratio larger than 1 has been found to improve the deformability index of inclusions due to formation of a film of liquid around it, rich in Te
 It has been reported that addition of 0.05% Te to leaded steels improves the machinability over the leaded once only
 Corus Steel
 have optimized sulphur content which balances the conflicting benefit of low sulphur for fatigue properties and high sulphur for improved machinability. Thus it may be inferred that Low sulphur level is essential for attaining improved fatigue properties. They have used SAE 1548 modified for crankshaft. However chemistry has not been given. One patent on rail steel
 consists of C 0.5-0.75%, 0.1-0.5% Si3O.9-1.5% Mn , and Te content of less than 0.004% & a sulphur content such that Te/S ratio is 0.1-0.6 It has been reported
 that when Tellurium is present in free machining steel , all the tellurium is in inclusion form, principally as lead telluride (PbTe) when lead is also present. Some of the tellurium may be present as manganese telluride. In conventional free machining , lead is normally present in the range of 0.25-0.35% and Tellurium is normally present in the range 0.04-0.06%.ln the absence of lead, Tellurium is present as bismuth telluride (if bismuth is added) or as manganese telluride (MnTe) which may be in the form of eutectic with manganese sulphide (MnS). Sulphur content in such steel is 0.25-0.35%.
Micro-alloyed steel in one of the patents  for improved fatigue resistance employs the following preferred compositions:
02-0.5% C, 1-1.5% Mn , 0.05-0.15% V and S 0.001 %-0.06%.
They have compared the endurance limits for SAE 1548 and TMS 80 micro-alloyed steels for crankshaft application. It may be noted that increasing S level has a detrimental effect on fatigue strength.
||Endurance Limit (Mpa)
|TMS80 Low s
|TMS 80 Med S (0.02%)
|TMS80 High s